Method of and bath for producing microcrack chromium coatings

ABSTRACT

A MICROCRACK CHROMIUM COATING IS APPLIED BY ELECTROPLATING IT UPON A COATING OF KICKEL, COLBALT NICKEL-COLBALT NICKEL-IRON OF IRON-COLBALT PEVIOUSLY ELCTRODEPOSITED UPON A SUBSTRATE, E.G., OF STEEL, FROM AN ELECCTROPLATENG BATH CONTAINING 5 G./LITER TO LIMIT OF SOLUBILITY OF AT LEAST ONE AROMATIC MONO-, DI- OR POLYCARBOXYLIC ACID AMYHDRIDE OR SALT. PREFERABLY THE AROMATIC CARBOXYLIC ACID IS BENZOIC ACID, PHTALIC ACID OR PHENYLACTIC ACID AND IS PRESENT IN AN AMOUNT OF 5 G./LITER TO 70 G./LITER OR THE LIMIT OF SOLUBILITY WHICHEVER IS LESS, THE ELECTROPLATING IS CARRIED OUT A TEMPERATURE OF 35 TO 70*C. AND THE CURRENT DENSITY OF 3 TO 20 A./DM.2.

United States Patent Office 3,753,872 Patented Aug. 21, 1973 3,753,872 METHOD OF AND BATH FOR PRODUCING MICROCRACK CHROMIUM COATINGS Martin Kohl, Dusseldorf, and Ralf Ludwig, Neuss,

Germany, assignors to Langbein-Pfanhauser Werke AG, Neuss, Rhine, Germany No Drawing. Filed Jan. 10, 1972, Ser. No. 216,724 Claims priority, application Germany, Jan. 11, 1971,

P 21 971.2 Int. Cl. C23b /08, 5/32, 5/50 US. Cl. 20441 5 Claims ABSTRACT OF THE DISCLOSURE A microcrack chromium coating is applied by electroplating it upon a coating of nickel, cobalt, nickel-cobalt, nickel-iron or iron-cobalt previously electrodeposited upon a substrate, e.g. of steel, from an electroplating bath containing 5 g./liter to the limit of solubility of at least one aromatic mono-, dior polycarboxylic acid anhydride or salt. Preferably the aromatic carboxylic acid is benzoic acid, phthalic acid or phenylacetic acid and is present in an amount of 5 g./liter to 70 g./liter or the limit of solubility whichever is less, the electroplatinng is carried out at a temperature of 35 to 70 C. and the current density of 3 to a./dm.

(l) FIELD OF THE INVENTION (2) BACKGROUND OF THE INVENTION Various systems for protecting corrodible metals such as iron and steel, have been provided heretofore and may make use of electroplating techniques to deposit one or more layers of corrosion-resistant metals having an esthetic character which is more desirable than that of the corroded substrate. In the automotive industry, for example, various parts of an automobile are given a microcrack chromium-plating to resist corrosion and improve appearance of metal substrates which have high structural strength. For some time it has been the practice to apply the microcrack coating upon an underlaying chromium coating which, in turn, is applied to the substrate. The microcrack chromium coating has the advantage of excellent corrosion-protection characteristics and these characteristics are enhanced when it is formed as the final layer in a corrosion-resistant system consisting of copper-nickel-chromium or nickel-chromium upon steel or iron bodies and upon pressure-cast zinc. The copper and nickel coatings may be deposited from a combined electrolyte or from individual electrolytic baths.

In the production of microcrack chromium coatings according to conventional techniques, the chromium layer is applied to a bright-nickel electrodeposited coating and is found to have relatively high internal stress and relatively low elongation-to-break valves. An electrolyte must be used which provides high dispersion if the optimal plating-thickness distribution is to be reached. The chromium coating is then electroplated with a final chromium coating from a second chromium-plating bath. The quality of such electrodeposited layers often leaves much to be desired and they have low bonding-strength tolerance to mechanical distortion. Similar chromium coatings which are rendered microporous by incorporating nonconductive particles in the underlying nickel coating, have similar disadvantages. It should be noted that efforts have been made to incorporate aliphatic organic acids or their metal salts in plating baths for high-stress nickel coatings but these systems also have disadvantages. For example, the nickel coating is not a high-internal-stress layer (a stress of the order of 980 N/mm. being common) while the elongation-to-break or failure load is about 0.5 and, therefore, relatively high. The anticorrosion character of such nickel coatings, tested in accordance with German Industrial Standards DIN 50,958 without kaolin or in accordance with ASTM standard B380, against a saturated calomel electrode in a Corrodkote solvents, revealed a potential of the layer of 300 to 500 mv. which is unsatisfactory. A further disadvantage of this latter system is that high current densities during electrodeposition result in clouding of the deposit, especially when the substrate is exposed to the plating conditions for a long period.

(3) OBJECTS OF INVENTION The principal object of the present invention is to provide an improved electroplating bath for the electrodeposition of an intermediate nickel layer upon a substrate such that the subsequent chromium deposit Will have a high and uniform microcrack character.

It is another object of the present invention to provide a nickel-plating bath for the purposes described which will avoid the aforementioned disadvantages.

Yet another object of the invention is to provide an improved method of applying corrosion-resistant coatings to metallic substrates.

It is also an object of our invention to provide an improved method of applying a microcrack chromium layer to substrates such as iron and steel.

(4) DESCRIPTION OF THE INVENTION The foregoing objects and others which will become ap parent hereinafter, are attained in accordance with the present invention, in a system for coating metallic substrates, especially bodies of iron, steel or pressure-cast zinc, with an intervening or first coating of a nickel or cobalt-containing layer from an electroplating bath containing at least one aromatic carboxylic acid, anhydride or metal salt, and thereafter applying the finishing layer from a chromium-plating bath upon this intermediate layer. The microcrack chromium layer may be applied as described in 'French Patent 1,447,970, or Dettner Elze, Bd.II, S. 184-188 (Hanser Verlag, Munich, 1966).

More specifically, the invention resides in the provision of an electrolyte or electroplating bath for the formation of the intermediate layer, which contains nickel, cobalt, nickel-and-cobalt, nickel-and-iron or iron-and-cobalt ions, the usual ammonium and chloride ions common in nickel and cobalt-plating baths and the customary additives thereto together with at least one aromatic mono-, dior polycarboxylic acid or a salt thereof (or the anhydride thereof which is converted to salt or free acid upon addition).

Preferably, the intervening coating is a nickel layer deposited from a nickel electroplating bath of conventional composition except for inclusion of at least 5 grams per liter (g./l.) of the aromatic carboxylic acid which can be present in amounts up to the limit of solubility thereof in the aqueous electroplating bath. Preferably, the aromatic carboxylic acid is benzoic acid, phthalic acid or phenylacetic acid and in the case of phthalic acid may be of the auto-, meta-, or paraform. Preferably the aromatic carboxylic acid is present in an amount between 5 to 70 g./l. or the limit of solubility whichever is less. More preferably, the aromatic carboxylic acid is present in an amount between 20 to 60 g./l.

As noted, the nickel-plating bath (or the cobalt-containing plating bath as the case may be) will contain the ususal substances employed in the deposition of the respective coatings. Reference is made in this respect to the Encyclopedia of Electrochemistry, Reinhard Publishing Corporation, New York, 1964. The metal ions are present in an amount ranging from 30 to 100 g./l. and will be nickel and cobalt and combinations thereof with one another or with iron, the metal ions being introduced into an aqueous bath in the form of the sulfate, sulfamate, chloride, fluorborate, fluoride and/or acetate. The bath further should contain 10 to 150 g./l. of chloride, introduced in the form of nickel, cobalt, iron, ammonium, magnesium or alkali-metal (lithium, sodium, potassium) chloride. 10 to 100 g. of ammonium ion may also be present and, if desired, boric acid in conventional quantities.

It has been found that best results are obtained during the plating of the intervening layer with current densities of 3 to 20 amperes per square decimeter (a./dm. preferably -10 a./dm. at temperatures of 35 to 70 C., preferably 40 to 50 C., pH values of 2.5 to 6, preferably from 3.5 to 3.9, and with plating times of 1-10 minutes, preferably 1 to 3 minutes.

The chromium coating is applied in accordance with the conventional technique described above. It is found that the system of the invention can operate at high current densities without significant clouding of the intermediate layer, that in all cases Within the temperature range of the present invention there is a volatilization of liquid from the bath so that replenishing chemicals can be added without increasing the volume, that the intermediate coating has an internal stress above 1960 N/mm. and an elongation to break of less than 0.1%. The potential of the intervening layer, measured against a saturated calomel electrode in accordance with DIN 5 0,95 8 and ASTM B380 as described above is between -100 and l50 mv., and thus is not disadvantageous from the point of view of corrosion protection.

(5 SPECIFIC EXAMPLE A steel-sheet substrate is coated with nickel in an aqueous bath of the following composition:

G./ liter Nickel 50-60 Ammonium ion 20-30 Chloride ion 90-100 Terephthalic acid 30-50 Electrodeposition was carried out for a period of three minutes at a pH of 3.0 to 3.9, a temperature of 40 to 50 C. and a current density of 5 to 10 a../drn. against the nickel electrode. The unclouded nickel layer was washed and coated with chromium in an aqueous bath containing 300 g./liter chromic acid, 2.5 g./1iter sulfuric acid and 1-3 g./liter aluminum silicofluoride. The chromium plating was carried out at a temperature of 30 C. and a current density of 6 a. ldm. against a chromium electrode for a period of 3 minutes. The nickel coating was found to have an internal stress value of 1960 N/mm? and a potential of -100 mv. measured as described above. The elongation at break was about 0.09% and the chromium coating had a crack count of 300-850 cracks per cm. uniformly distributed over the electroplated region.

Similar results were obtained when 50% by weight of the nickel was replaced by cobalt, when cobalt was substituted entirely for the nickel and when 10% of the nickel was replaced by iron and when nickel was replaced by c0balt+10% iron.

In general, the chromium-plating step is carried out using a bright-chromium plating bath under cold conditions as described in the aforementioned publications at a temperature of 25 to 35 C. and a current density of 3 to 12 a./dm.

What is claimed is:

1. A method of coating a metal substrate to restrict corrosion thereof, comprising applying a first metallic layer to said substrate by electroplating nickel, cobalt or a combination thereof, or a combination of either with iron onto said substrate from a aqueous acidic electroplating bath containing the respective metal ions, ammonium ion and chloride ion together with 20 to 60 g./l. of phthalic acid and thereafter electrodepositing a chromium layer upon said first layer to produce a microcrack coating.

2. The method defined in claim 1 wherein said first layer is electrodeposited from a bath containing 30 to 50 g./liter of phthalic acid for a period of 1 to 3 minutes at a pH of 3.5 to 3.9 and a temperature of 40 to 50 C. with a current density of 5 to 10 a./dm.

3. The method defined in claim 2 wherein said chromium layer is applied by electroplating from a bright chromium cold bath at a temperature of 25 to 35 C. and a current density of 3 to 12 a./dm.

4. The method defined in claim 1 wherein said first layer is a nickel layer.

' 5. An aqueous acidic electroplating bath containing essentially 30 to g./l. of nickel or cobalt ion individually or in combination, or in combination either with iron ion, 10 to g./1. chloride ion, 1 to 100 g./liter ammonium ion and 20 g./1. to 60 g./l. of phthalic acid.

References Cited UNITED STATES PATENTS 3,152,009 10/1964 De Long 1l7130 E 3,488,264 1/ 1970 Bailey et al 20449 2,748,068 5/1956 Faust et al. 20443 2,712,522 7/1955 Kardos et a1 20449 3,264,200 8/1966 Clauss et a1 20449 3,417,005 12/1968 Baig 20449 X 3,563,864 2/1971 Du Rose et a1 20441 X 3,574,067 4/1971 Spiro 20449 X FOREIGN PATENTS 1,006,333 9/1965 Great Britain 20449 2,465 6/1880 Great Britain 20449 OTHER REFERENCES Frederick A. Lowenheim, Modern Electroplating, p. 289 (1968).

GERALD L. KAPLAN, Primary Examiner U.S. Cl. X.R. 20443 T, 48, 49 

